You can use Square to handle the payment processing on your SimpleTix website.
Find this at: Settings & Options > Payment Providers.

On the very top of this list you'll see Square. 
Click it to reach your Square settings page.

If you have not already connected your site to Square, click the Connect with Square button.

Once your site is connected, all customer's orders will be processed with Square and the funds will go directly to your Square account.

Want to use the Square register (iPad, iPhones, iPods, and Android) at your event?

Here's the steps:

Make the Square connection (above).
Then click the checkbox to enable the Square Register.
Ensure that the location you pick the location where you would like to load your SimpleTix types.

e.g. if you have a downtown brewery, you may want to create a different location for your craft beer festival.

From your SimpleTix manager's panel, go to Manage > Events.
Navigate to an event's ticket page. You will now need to push each ticket type that you would like to sell on-site to your Square Register.
On the list of ticket types notice the new column related to Square. This indicates whether you have pushed this ticket type to your Square register.

Click Save to push the ticket type to your Square register. From this dialog box you have the ability to adjust the name and ticket price.    
If you plan to use Square on a smart phone, we recommend to keep the name as short as possible so it appears best on your Square device. If you plan to sell tickets for a higher price on-site at your event, you can set a higher price in this dialog box, it will not effect your website sales.
After you save each ticket type, you should see it on your Square register.

It's the day of your event! Let's sell the tickets using the Square Register!

Here's how the process works. Keep in mind, while there are a few extra steps than normal, once you do it a few times it's becomes second nature.